Colloquium: Boulat Bash

    Thursday, November 30, 2017 - 3:30pm - 5:00pm
    Meinel 307

    Hiding Signals in Noise: Fundamental Limits of Covert Communication and Sensing


    Hiding transmitted signals is of paramount importance in many communication and sensing settings. In communications, traditional security (e.g., encryption) prevents unauthorized access to message content; however, detection of the mere presence of a message can have significant negative impact on the privacy of the communicating parties. Unlike these standard methods, communicating using covert or low probability of detection/interception (LPD/LPI) signals not only protects the information contained in a transmission from unauthorized decoding, but also prevents the detection of a transmission in the first place. Similarly, performing active sensing tasks requires the use of covert signaling waveforms to prevent the adversary from detecting the sensor’s transmitter.

    In this talk I will focus on covert communication. I will begin by providing a general overview of the subject. I will then develop the fundamental laws of covert communication. First, I will show the results for covert communication over additive white Gaussian noise (AWGN) channels, a standard model for radio-frequency (RF) communication. I will present a square root law that governs how much information can be transmitted covertly and reliably over such channels. Specifically, I will prove that if the transmitter has the channels to the intended receiver and the warden that are both AWGN with constant noise power, then O(√n) covert bits can be reliably transmitted to the receiver in n uses of the channel. Conversely, attempting to transmit more than O(√n)) bits in this scenario either results in detection by the warden with probability one or a non-zero probability of decoding error at the receiver asn→∞. I will then demonstrate how interference and fading in practical RF channels can allow the reliable transmission of O(n) covert bits in n channel uses, and show a jammer-assisted scheme to achieve this significantly improved rate of covert communication.

    I will then transition to covert optical communication and characterize the ultimate limit of covert communication that is secure against the most powerful physically-permissible adversary. I will show that, although covert communication is impossible when a channel injects the minimum noise allowed by quantum mechanics, it is attainable in the presence of any noise excess of this minimum (such as the thermal background). In this case, the square root law governs covert communication, as in the AWGN channel case. Moreover, this scaling is achievable using standard optical communication equipment, even when all-powerful adversary may intercept all transmitted photons not received by the intended receiver, and employ arbitrary quantum memory and measurements. Conversely, I will show that the square root scaling cannot be circumvented. I will then present a basic proof-of-concept experiment on an optical testbed that corroborates our theory.

    Finally, I will overview our results on the limits of active covert sensing. I will focus on sensing an unknown phase of a single pixel using optical probes. These probes pass through the same channel as in the covert optical communication scenario. I will prove that the performance of covert active sensing is also governed by a square root law: the mean squared error of an unbiased estimator that employs n-mode optical probes scales as O(1⁄√n). I will fully characterize this limit and will show that it is achievable using laser light illumination and a heterodyne receiver, even when the adversary captures every photon that does not return to the sensor and performs arbitrarily complex measurement as permitted by the laws of quantum mechanics.


    Speaker Bio(s): 

    Boulat Bash, Ph.D., is a Scientist with the Quantum Information Processing Group, Raytheon BBN Technologies. He received the B.A. degree in economics from Dartmouth College in 2001 and the M.S. and Ph.D. degrees in computer science from the University of Massachusetts, Amherst, MA, USA, in 2008 and 2015, respectively. His research interests include security, privacy, communications, signal processing, and information theory. He received the University of Massachusetts School of Computer Science Outstanding Doctoral Dissertation Award (2015) for his thesis on the fundamental limits of covert communication.